Two-stage electrohydraulic servovalves (sometimes called "proportional control" valves) are in common use in industrial applications to control the flow of fluid, or pressure, with respect to a load. These are typically used to control the position of a load in response to a command signal.
Some applications require a "fail-safe" behavior of the valve such that, when either supply pressure or electrical power is lost, flow through the output-stage will be blocked. To provide this behavior, the second-stage valve spool may be mechanically centered by springs, which function to return the spool to a centered or null position in the absence of supply pressure or electrical power. In some cases, a bypass circuit is provided to equalize the pilot-stage output pressures in the event of an electrical failure, so as to allow the centering springs to return the second-stage spool to its centered or null position. The second-stage valve spool typically has lobes which are overlapped with respect to control ports so that there will be minimum leakage from the supply pressure to the load, or from the load to the return, when the valve spool is in its centered or null position.
The provision of overlapped spool lobes introduces significant non-linearity in the second-stage output flow-to-command signal characteristics of the valve, particularly if second-stage valve spool displacement is a linear function of the input current, as is customary with flow-control servovalves. The output flow vs. input current characteristics of zero-lapped, overlapped and under-lapped spool lobes are comparatively shown and described in U.S. Pat. No. 4,766,921, the aggregate disclosure of which is hereby incorporated by reference. In many cases, it is desired to provide an overlapped spool lobe with respect to a control port, so as to minimize leakage flow through the output stage, particularly when the load is to be held statically for long periods of time. At the same time, it would be generally desirable to improve the output flow-to-command signal characteristics of such valve in order to reduce and minimize, if not substantially eliminate, the effect of the "dead zone" attributable to such overlapped spool lobes.